Communication means roughly fall into two distinct categories:
the busses, allowing data transfers in parallel form. They include data lines (e.g. 16 lines for transfers by halfwords) and control lines to sustain the protocol. Busses are well suited for applications needing fast data transfers over limited distances.
However, the number of wires is high (e.g. 30 lines), and the port attaching each communicating part is costly. On the other hand, the protocol used on busses is generally simple.
the serial links, allowing data transfers in serial form. Only 1 or 2 links (i.e. 2 or 4 wires) are needed to communicate at more moderate data rates over large distances. However, as every control and data information is transmitted on the same link, a sophisticated protocol is needed (such as HDLC protocol), and thus the port attaching each communicating part is also costly.
None of the above solutions is technically nor economically appropriate in case of a control link between a control unit and devices.
In fact, in such a case, several requirements have to be met: a small amount of information has to be exchanged between the control unit and the devices on a very reliable basis, at moderate transmission rates and over moderate distances.
It is then an object of the invention to provide a simple control link structure having only a limited amount of complexity contained in each device port, and wherein the link includes only a limited number of wires.
It is a further object of the invention to provide a control link structure and a protocol enabling each device to be connected either in a point-to-point or a multi-point configuration, and wherein it is further possible to efficiently test the whole link from the control unit, thus enhancing reliability.